The automotive industry, in order to reduce the weight of automobiles, has increasingly substituted aluminum alloy panels for steel panels. Lighter weight panels help reduce automobile weight, which reduces fuel consumption. However, the introduction of aluminum alloy panels creates its own set of needs. To be useful in automobile applications, an aluminum alloy sheet product must possess good forming characteristics in the as-received T4 temper condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling. At the same time, the alloy panel, after painting and baking, must have sufficient strength to resist dents and withstand other impacts.
In addition to the requirements discussed above, another requirement is that the aluminum alloys for automotive components do not have objectionable and/or deleterious surface defects referred to as ridging, roping, or paint brush lines, which appear on the surface of stamped or formed aluminum sheet components. The ridging or roping lines appear in the rolling direction only upon application of sufficient transverse strain, such as that occurring in typical stamping or forming operations.
This roping defect is of sufficient severity that it is visible in the automotive component after painting. Consequently, the finished surface appearance of these aluminum alloys is objectionable and not suitable for exterior automotive applications. This roping defect may also serve as a strain concentration site during forming, thus limiting formability.
In addition, known processes for making 6xxx series sheet material suitable for automotive outer panels have involved a rather complex, expensive, and slow procedure generally involving the following steps: semi-continuous direct chill (DC) casting of the molten alloy to form an ingot, scalping of the ingot, homogenizing the ingot for time periods between 1 to 48 hours, hot rolling, self-annealing, and cold rolling to the desired gauge. The rolled material may then be given a solution heat treatment in a continuous heat treatment line, rapidly cooled and then aged.
Typically, the self-anneal process for automotive outer panels includes high exit temperatures which are required to meet roping requirements. High exit temperatures promote large soluble coarse particles, such as Mg2Si and copper containing particles. To achieve the desired combination of strengths in the as-supplied and paint bake tempers, continuous anneal solution heat treatment (CASH) lines must use high solutionizing temperatures and long soak times to dissolve the large soluble particles. These soluble particles are known to affect tensile properties in both T4 and paint bake tempers and also forming characteristics. The large soluble particles can ultimately decrease CASH line productivity to an unacceptable level. Additionally, the self-annealing process reduces the ability to differentiate alloys in terms of both T4 and paint bake strengths, despite significant differences in the alloy chemistries.
Inner and structural automotive products are generally produced from rerolls that are coiled at relatively lower temperatures. These products meet the tensile properties but not the roping requirements of outer products using lower CASH peak metal temperatures and soak times. Thus, metal sheets for inner/structural panels tend to run between 25-50% faster on the CASH line than metal sheets for outer panels. In this overall procedure, the processing of outer panels for automotive applications can take a substantially long solutionizing time, which reduces productivity of an expensive asset.
There is therefore a need for improved aluminum alloys that have reduced roping defects and for more efficient processes for fabricating sheet material from such alloys.